Captain Nixon continue drinking even though he has to hide his alcohol because he is addictive to drinking and can't resist to it. he does not get into trouble for his drinking because he is the highest authority on the ship and nobody can question him for his actions. If he is of lowering rank, then the higher authority on the ship can question him and can stop him from drinking alcohol on the duty. So that's why he continue drinking and not get into trouble for his drinking.
